Mmm, I wrote the issue thinking web library displayed pdf's with the zotero reader, but I deduce from your comment that it is displayed with the browser pdf reader. I am using the Zen browser with the Windows default theme, dark.
No, the PDF is displayed using the Zotero Reader; however, both the browser and the OS are relevant here because our code ultimately depends on both to render the PDF content.
Zen Browser is based on Firefox, so it should work, and in fact, it does work for me on macOS (see below). I suspect this might be related to the fonts on your system or something specific to Zen Browser on Windows. I'd recommend trying one of the mainstream browsers (Chrome/Firefox/Edge) to confirm.
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What browser and operating system are you using? Did you try re-downloading this particular PDF to ensure it's not corrupted in any way?
Zen Browser is based on Firefox, so it should work, and in fact, it does work for me on macOS (see below). I suspect this might be related to the fonts on your system or something specific to Zen Browser on Windows. I'd recommend trying one of the mainstream browsers (Chrome/Firefox/Edge) to confirm.